The invention relates to a temperature controller for a photographic heat fixing unit, and more particularly, to such a controller used in an electrophotographic copying apparatus in which a temperature sensor detects the temperature of a heat fixing unit and produces an output which is utilized to control the operating temperature of the unit.
A photographic heat fixing unit which is used in an electrophotographic copying apparatus to fix a toner image generally comprises an open heater fixing unit internally housing a ribbon-shaped heater or a wire heater, a radiant heater fixing unit which utilizes a halogen lamp or an infrared lamp, or a heat roll fixing unit which utilizes heated rolls. It is mandatory that the operating temperature of such fixing units be controlled to a point where a satisfactory fixing operation is assured. To this end, such a unit includes a temperature sensor and an associated control circuit. However, in conventional arrangements for controlling the temperature of described heat fixing units, although the temperature of the location where a temperature measurement is made can be maintained constant, such temperature is not always in coincidence with the temperature which is required to achieve a toner fixing, thereby causing various drawbacks. By way of example, the fixing situation which prevails immediately after the power switch of the fixing unit has been turned on is different from the fixing situation after a number of copies have been produced in succession or after the unit has been maintained in its standby mode while heating it over a prolonged period of time. It is sometimes possible that a record sheet may be scorched or burnt before or after the fixing operation. Additionally, an improper fixing temperature may cause a so-called offset of a toner image. As is recognized, a proper fixing condition greatly depends on the thickness and the hygroscopic degree of a record sheet, but a conventional fixing unit failed to provide a satisfactory control.
One form of electrophotographic copying apparatus in which a heat fixing unit is incorporated will now be described with reference to FIG. 1. The apparatus is shown in its normal condition in which a single sheet-shaped original is adapted to be fed along its conveying path. A sheet-shaped original 1 is placed on an inclined original guide 2 and is inserted into an inlet opening of an original feeder in a direction indicated by an arrow A, the feeder comprising pairs of conveying rollers 4A, 4B, 5A, 5B and a pair of guide plates 6, 7. As the original 1 moves into the nip between the pair of rollers 4A, 4B, it is conveyed toward an exposure station 8. It then passes between the pair of guide plates 6, 7 and between the guide plate 6 and the exposure station 8. After passing through the exposure station 8, the original 1 is further conveyed by the pair of rollers 5A, 5B onto an original tray 9.
As the original 1 is conveyed by the original feeder, a pair of microswitches 3A, 3B, located on the opposite sides of the conveying rollers 4A, 4B, operate to detect the location of the original 1 and produce outputs which are utilized for controlling the timing of operation of various parts of the apparatus. As the original 1 passes through the exposure station 8, an illumination lamp 10 projects light onto the surface of the original, whereby an optical system 11 projects an optical image of the original onto a photosensitive drum 12. The drum 12 is adapted to rotate in a direction indicated by an arrow B, and is electrically neutralized by a neutralizer lamp 13 and is then uniformly charged by a corona charger 14 before it is irradiated with the optical image of the original to thereby form an electrostatic latent image of the original 1 on its surface. The latent image is developed by a developing unit 15 of the dry type, and is then carried to a toner image transfer station 16 as the drum 12 continues to rotate. On the other hand, a number of record sheets 18 are contained in a stack within a cassette 17, and are fed one by one by an oscillating and rotating feed roller 19 so as to be conveyed to the transfer station 16 by a pair of vertically spaced feed rollers 20 at a given timing. The record sheet 18 is conveyed into the nip between the drum 12 and a transfer roller 21, to which a bias voltage is applied, so that the sheet is brought into overlapping relationship with the toner image on the drum 12, thus allowing the toner image to be transferred to the sheet. Since it is possible that the record sheet be carried by the drum 12 while it is in close contact therewith, the sheet is separated therefrom by the action of a separating claw 22 and an airstream to be described later. After the toner image is transferred to the record sheet, the latter is then conveyed along a guide 23 and is then fed by a pair of vertically aligned feed rollers 24 into a heat fixing unit 25 of the open type where the toner image is melted and fixed to the sheet. Subsequently, the sheet is delivered onto a copy tray 27 through an outlet 35, by the action of a pair of vertically aligned delivery rollers 26.
Any small amount of residual toner which remains on the drum 12 without being transferred to the record sheet is scraped off by a rotating cleaning brush 28 and withdrawn into an airstream created by a fan 29 to be collected by a filter 30. Both the cleaning brush 28 and the fan 29 are covered within a case 31 in order to produce an effective displacement of the residual toner and to prevent a dispersion of the toner within the apparatus. The airstream displaced by the fan 29 is introduced into a duct 32 which has its outlet port 32a located adjacent to the transfer station 16 so that it can be utilized to achieve an effective separation of the record sheet from the drum 12 by cooperation with the separating claw 22.
The illustrated electrophotographic copying apparatus is arranged to provide a plurality of copies in succession through the repetition of a developing of the electrostatic latent image once formed on the drum 12 with toner and a subsequent transfer step. When the apparatus is so used, the cleaning brush 28 mounted on a holder 34 which is pivotally mounted on a pin 33 is moved away from the drum 12 and both the neutralizer lamp 13 and the charger 14 are disabled.
FIG. 2 is a cross section showing the heat fixing unit 25 used in the described electrophotographic copying apparatus in more detail while FIG. 3 is a plan view taken along the line X--X shown in FIG. 2. In FIGS. 2 and 3, there are shown a pair of containers 41, 42 in the form of metal boxes which are open at their one end and internally housing band-shaped heaters 43, 44, respectively, which are mounted on support members 47a, 47b and 48a, 48b, respectively. The containers 41, 42 are disposed in spaced, opposing relationship with each other on the upper and the lower side of a path 51 along which the record sheet carrying the transferred toner image is conveyed, with their openings 41a, 42a facing toward each other. The support members 47a, 47b, 48a, 48b are formed of an electrically insulating, refractory material such as ceramics, and are secured to the top wall of the container 41 and the bottom wall of the container 42 along their opposite lateral ends. A plurality of band-shaped heaters 43, 44 extend across the support members 47a, 47b and 48a, 48b.
These heaters 43, 44 are formed as strips, having a sheet thickness on the order of 0.05 to 0.2 mm, of an electrical resistance material such as iron chrome, nickel chrome or the like, and may be painted with a black paint, if required. It will be noted that the heaters 43, 44 are disposed toward the respective openings 41a, 42a of the containers 41, 42 and have their opposite ends secured to steps formed in the individual support members 47a, 47b, 48a, 48b by set screws 45. When the plurality of heaters 43, 44 are secured to the support members by set screws 45, they are connected in series with each other by conductive strips 46 disposed on the individual support members. The series combination of heaters 43, 44 are connected through a switch to an a.c. source, not shown.
A plurality of guides 49, 50 in the form of thin wires extend across the openings 41a, 42a of the containers 41, 42 generally in a direction parallel to the direction of movement of the record sheet. As shown, the guides 49, 50 are disposed to be further spaced from each other, as viewed in a direction indicated by an arrow B.sub.0 or in the direction of movement of the record sheet, thereby assuring a smooth conveying of the record sheet without causing a jamming in the path 51. Referring to FIG. 2, it will be noted that the pair of feed rollers 24 and the pair of delivery rollers 26, disposed on the opposite sides of the path 51 extending through the fixing unit forcedly convey the record sheet along the path 51.
Heat resistant members 52, 53 are applied to the inside of the top wall of the container 41 and of the bottom wall of the container 42 to provide a heat insulation and an electrical insulation. Heat insulating plates 54, 55 are applied to the external surface of the top wall of the container 41 and of the bottom wall of the container 42.
A temperature sensor 56 such as a thermistor is disposed close to or in contact with one of the heaters 43 in the photographic heat fixing unit 25 for detecting the operating temperature of the heater. By providing such a sensor, a temperature controller is associated with the fixing unit 25 for controlling the operating temperature of the heaters 43, 44 to a given value. The fixing unit 25 shown exhibits a rapid temperature rise upon energization of the heaters 43, 44, and permits a fixing operation to be initiated within ten to fifteen seconds. The temperature control of the heaters 43, 44 has been performed by utilizing a temperature controller as shown in FIG. 4.
FIG. 4 is a circuit diagram of one form of conventional temperature controller. The controller includes a pair of terminals 70, 71 across which a d.c. voltage of a given magnitude is applied. A thermistor S.sub.1 or the temperature sensor 56 which detects the operating temperature of the heater 43 is connected in series with a resistor 72 across the terminals to form a circuit which produces a detection signal. A reference voltage circuit is formed by a series combination of a variable resistor 73 and a fixed resistor 74, and is connected in parallel with the sensor circuit. The potential appearing at a junction P between the thermistor S.sub.1 and the resistor 72 and the potential appearing at a junction P.sub.0 between the variable resistor 73 and the resistor 74 are introduced into a comparison and detection circuit 75 where they are compared against each other. The reference voltage at the junction P.sub.0 can be preset to a proper value by adjusting the variable resistor 73. The comparison and detection circuit 75 produces an output which is effective to reduce the potential difference between the junction P, P.sub.0. The output is applied to an amplification control circuit 76 which controls the energization of the heaters 43, 44. FIG. 5 shows the circuit arrangement more specifically. The comparison and detection circuits 75 comprises an operational amplifier 77 combined with resistors 78 to 81. The inverting input terminal of the amplifier 77 is connected to the junction P while the non-inverting input terminal is connected to the junction P.sub.0. A feedback resistor 78 is connected across the non-inverting input terminal and the output terminal of the amplifier 77. A series combination of resistors 79, 80 and 81 is connected across the terminals 70, 71, with the junction between the resistors 79, 80 being connected to the output terminal of the amplifier 77 and the junction between the resistors 80, 81 being connected to the input of the control circuit 76, which is also connected with the terminal 71. The control circuit 76 is arranged to control a power supply 82 associated with the heaters 43, 44.
It is to be noted that the temperature control of the fixing unit is achieved by merely detecting the operating temperature of the heaters 43, 44. By experiment, it has been recognized that the following problems are presented when the heat fixing unit 25 is controlled by utilizing the temperature sensor 56 alone:
(1) When a fixing operation is performed shortly after the energization of the fixing unit which has reached a sufficient temperature to permit a fixing operation, the amount of heat generated is significantly less than that produced during a fixing operation which is performed after a number of copies have been produced in succession. Consequently, a dissatisfactory fixing may result when initiated shortly after the energization of the fixing unit.
(2) When a number of copies are produced in succession, the temperature of the record sheet or delivery rollers 26 may rise excessively, causing a scorching of the record sheet or an offset of the toner image by the delivery rollers 26.
(3) When a thick record sheet or a relatively humid record sheet is used, the fixing result may be insufficient. On the other hand, when a thin or a dry record sheet is used, a satisfactory fixing operation may be achieved, but leaves the likelihood that the record sheet may be scorched or an offset of the toner image may be caused by the delivery rollers 26.
These problems cannot be overcome with the conventional temperature control system in which the heaters 43, 44 are energized to a given level and maintained at such level. The problems mentioned under subparagraphs (1) and (2) are attributable to the fact that while a given temperature is maintained at the location where the temperature is sensed other members of the fixing unit 25, for example, the containers 41, 42, the support members 47a to 48b or the like exhibit a temperature rise at a rate which is greatly lower than that of the heaters 43, 44 and that the temperature rise of members other than the heaters 43, 44 may also contribute to the heating of the record sheet.
The problem mentioned under the subparagraph (3) is attributable to a variation in the heat capacity of the record sheet itself, and hence an essential solution cannot be attained unless the temperature control takes the heat capacity of the record sheet into consideration.
It will be evident from the foregoing description that these problems are present, not only in the heat fixing unit 25 of the open type mentioned above, but also are present in common in every heat fixing unit which is operatively used while its temperature is being controlled.
FIG. 6 is a cross section of one form of heat roll fixing unit which is known in itself. A fixing unit 66 comprises a pair of rolls 60, 61 which are formed by cylindrical metal pipes and which are provided with coatings 62, 63 of polytetrafluoroethylene, silicone rubber or the like on the surface of the rolls. A heater 64 is disposed inside the roll 61. The temperature control of the heat roll fixing unit 66 is achieved by utilizing a heat transfer strip 65 disposed in contact with the surface of the heat roll 61 and a temperature sensor 56 carried thereby. The heat roll fixing unit 66 exhibits a slow temperature rise, and allows a fixing operation to be initiated after a time interval from one to ten minutes upon energization thereof. Accordingly, it is normally energized so that a given temperature is maintained while it is in its steady mode. However, when a temperature is reached after it is initially energized, members located around the heat roll 61 still assume a low temperature while these members reach a higher temperature when the unit is in its standby mode or after a number of copies have been produced in succession. Consequently, a fixing result of the record sheet is dependent upon the heat of these members. With the heat roll fixing unit 66, an offset may be caused in a temperature region which is either higher or lower than the proper temperature, and hence a change in the fixing performance cannot be neglected. Obviously, the fixing performance also depends on the thickness of the record sheet as well as the hygroscopic degree thereof in the same manner as experienced with the fixing unit of the open type.